Mask filter and method of manufacturing the same, and particle-trapping mask

ABSTRACT

An aspect of the present invention relates to a mask filter, wherein wet nonwoven fabric comprised of synthetic fiber is folded up into a concentric circular pleat form, and the synthetic fiber comprises 10 to 50 mass percent of short fiber A having a fiber length ranging from 0.1 to 1 mm and a fiber diameter ranging from 100 to 1,000 nm, and further comprises binder fiber B having a fiber length ranging from 5 to 10 mm and a fiber diameter ranging from 6 to 15 μm, and fiber C of greater fiber length and fiber diameter than short fiber A and smaller fiber length and fiber diameter than binder fiber B.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese PatentApplication No. 2010-186867 filed on Aug. 24, 2010, which is expresslyincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a mask filter and to a method ofmanufacturing the same. More particularly, the present invention relatesto a mask filter affording both good particle filtration efficiency andlow pressure loss, and to a wet method of manufacturing the abovefilter.

The present invention further relates to a mask for trapping particlesequipped with the above filter.

BACKGROUND ART

In addition to sheet-like filters, filters with wave-shaped andconcentric circular pleats are known as the filters used inparticle-trapping masks such as dust masks. Among them, filters havingconcentric circular pleats afford desirable characteristics, such ashigh particle filtration efficiency and low pressure loss, as thefilters of particle-trapping masks.

Conventionally, from the perspective of suitability to manufacturing,filters having concentric circular pleats described above have beenmanufactured out of glass fiber, as described in Japanese ExaminedPatent Publication (KOKOKU) Showa No. 55-47929, which is expresslyincorporated herein by reference in its entirety. However, glass fiberis polydisperse and nonuniform in terms of fiber diameter and fiberlength. Thus, a complex performance-adjusting step is required to obtaina filter of constant particle filtration efficiency and airflowresistance from glass fiber.

In contrast, wet nonwoven fabric of synthetic fiber that can be used asa filter material in masks has been proposed in recent years (forexample, see WO2008/130019A1 or English language family memberUS2010/0133173A1, which are expressly incorporated herein by referencein their entirety). The use of synthetic fiber makes it possible toreadily achieve uniform fiber diameter and fiber length. Thus, the useof synthetic fiber permits the stable supplying of high-quality filters.Among synthetic fibers, short fibers such as those described inWO2008/130019A1 (so-called “nanofibers”) afford good sheet-formingproperties, and are thus suitable as materials for obtaining filters bywet methods.

Accordingly, the present inventors attempted to manufacture a filterhaving concentric circular pleats in which a wet nonwoven fabriccomprised of synthetic fibers containing nanofibers was folded up. As aresult, difficulty was encountered in obtaining a filter having pleatsof desired shape, and in some cases, no filter could be manufactured atall.

SUMMARY OF THE INVENTION

An aspect of the present invention provides for a filter made of wetnonwoven fabric comprised of synthetic fibers and having concentriccircular pleats.

The present inventors conducted extensive research into obtaining theabove filter, resulting in the discovery that by combining prescribedsynthetic fibers, it was possible to obtain a filter made of wetnonwoven fabric comprised of synthetic fibers that was folded up into aconcentric circular pleat form. The present invention was devised onthat basis.

An aspect of the present invention relates to:

a mask filter, wherein wet nonwoven fabric comprised of synthetic fiberis folded up into a concentric circular pleat form, and

the synthetic fiber comprises:

10 to 50 mass percent of short fiber A having a fiber length rangingfrom 0.1 to 1 mm and a fiber diameter ranging from 100 to 1,000 nm,

and further comprises:

binder fiber B having a fiber length ranging from 5 to 10 mm and a fiberdiameter ranging from 6 to 15 μm, and

fiber C of greater fiber length and fiber diameter than short fiber Aand smaller fiber length and fiber diameter than binder fiber B.

The above synthetic fiber can comprise 50 to 90 mass percent of a totalquantity of binder fiber B and fiber C.

The mask filter can have a DOP particle filtration efficiency of equalto or greater than 80 percent and a pressure loss of equal to or lessthan 300 Pa.

The mask filter can have a mass per unit area of 80 to 180 g/m².

The mask filter can have a filter thickness ranging from 0.4 to 1.5 mm,a pleat height ranging from 5 to 30 mm, a pleat outer diameter at theoutermost circumference position ranging from 50 to 120 mm, and a totalnumber of pleats ranging from 6 to 15.

The mask filter can have a total weight ranging from 2 to 12 g.

The short fiber A can be a polyester fiber.

The binder fiber B can be a core-in-sheath composite fiber with a coreportion of polyethylene terephthalate and a sheath portion of copolymerpolyester.

The fiber C can be a polyester fiber.

A further aspect of the present invention relates to:

a method of manufacturing the above mask filter, which comprises foldingup the nonwoven fabric into a concentric circular pleat form while in awet state.

A still further aspect of the present invention relates to:

a particle-trapping mask comprising the above mask filter.

The present invention permits the manufacturing by a wet method of amask filter of a concentric circular pleat faun affording good particlefiltration efficiency and low pressure loss.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of an example of the mask filter of thepresent invention.

FIG. 2 shows an example of the mask of the present invention.

MODES FOR CARRYING OUT THE INVENTION

The present invention relates to a mask filter in which wet nonwovenfabric comprised of synthetic fiber is folded up into a concentriccircular pleat form (also referred to hereinafter as a “mask filter” or“filter”). In the mask filter of the present invention, the syntheticfiber comprises of 10 to 50 mass percent of short fiber A having a fiberlength ranging from 0.1 to 1 mm and a fiber diameter ranging from 100 to1,000 nm, and further comprises binder fiber B having a fiber lengthranging from 5 to 10 mm and a fiber diameter ranging from 6 to 15 μm,and fiber C of greater fiber length and fiber diameter than short fiberA and smaller fiber length and fiber diameter than binder fiber B.

As set forth above, when the present inventors attempted to manufacturea filter having concentric circular pleats by folding up wet nonwovenfabric made of synthetic fiber comprising nanofibers, they found that itwas difficult to obtain a filter having pleats of desired shape and thatin some cases, no filter could be manufactured at all. This point willbe described in greater detail. In the course of wet manufacturing afilter having concentric circular pleats, synthetic fibers that had beensubjected to a sheet-forming process (nonwoven fabric) were folded upinto concentric circular pleats with a folding machine while wet, dried,and subsequently removed from the folding machine to obtain a filter.However, the synthetic fiber containing the nanofibers was difficult toseparate from the sheet-forming machine. When an attempt was made toremove the filter from the folding machine after drying, a phenomenonoccurred whereby a portion of the material stuck to the folding machineduring separation. Thus, it was difficult to obtain a filter of desiredshape.

The present inventors conducted further extensive research to discoverthe reason behind the above phenomenon. As a result, they found that itwas difficult to ensure the thickness and strength of minute nanofibers,and attributed the above phenomenon thereto. Accordingly, the presentinventors conducted further extensive research based on the aboveknowledge, resulting in the discovery that by combining prescribedsynthetic fibers with the nanofibers, it was possible to obtain a filterhaving concentric circular pleats of desired shape by a wet method. Thepresent invention was devised on that basis.

The mask filter of the present invention will be described in greaterdetail below.

Short Fiber A

In the synthetic fiber included in the filter of the present invention,10 to 50 mass percent of the fiber is a short fiber A having a fiberlength ranging from 0.1 to 1 mm and a fiber diameter ranging from 100 to1,000 nm. When the fiber diameter of short fiber A is greater than 1,000nm, the diameter of the holes that are present on the outer surfacebecomes nonuniform (that is, the ratio of the average hole diameter tothe maximum hole diameter becomes large), compromising filterperformance. At less than 100 nm, the fibers tend to drop out of thesheet-forming machine during sheet-forming, and it becomes difficult toobtain a filter of uniform thickness. Further, when the fiber length ofshort fiber A exceeds 1 mm, the fibers tend to become entangled anddispersion decreases during sheet-forming, making it difficult to obtaina filter of uniform thickness. When the fiber length of short fiber A isless than 0.1 mm, the grip between individual fibers becomes extremelyweak, making it difficult to move the nonwoven fabric from thesheet-forming machine to the folding machine. Further, from theperspective of filter moldability, the ratio of the fiber length L tothe fiber diameter D (L/D) desirably falls within a range of 100 to2,500, preferably within a range of from 500 to 2,000.

In this context, the fiber diameter referred to in the present inventioncan be measured by photographing the fiber cross-section at amagnification of 30,000-fold by a transmission electron microscope(TEM), and the fiber length that is referred to can be measured byobserving a fiber that has been placed on a base at a magnification of20 to 500-fold by a scanning electron microscope (SEM). In theseprocesses, TEM and SEM devices with length-measuring functions can beutilized to measure the fiber length and fiber diameter. For TEM and SEMdevices without length-measuring functions, it suffices to make anenlarged copy of a photograph that has been taken and measure the fiberdiameter and fiber length with a ruler adapted to the scale involved.

In this process, when the cross-sectional shape of a single fiber is adifferent shape from a round shape, the diameter of a circlecircumscribed onto the cross-section of the single fiber is employed asthe fiber diameter. In the present invention, the terms “fiber diameter”and “fiber length” refer to the average of the values measured for fivefibers.

Because the filter of the present invention is made of synthetic fibers,short fiber A is selected from among synthetic fibers. Examples of thesynthetic fiber included in short fiber A are polyamide fibers,polyester fibers, and polyolefin fibers. Desirable examples of polyesterfibers are polyethylene terephthalate, polytrimethylene terephthalate,polybutylene terephthalate, and polyethylene naphthalate; and copolymersof the above as primary repeating units with aromatic dicarboxylic acidssuch as isophthalic acid and 5-sulfoisophthalic acid metal salts,aliphatic dicarboxylic acids such as adipic acid and sebacic acid,hydroxycarboxylic acid condensates such as c-caprolactam, and glycolcomponents such as diethylene glycol, trimethylene glycol,tetramethylene glycol, and hexamethylene glycol. Desirable examples ofpolyamide fibers are aliphatic polyamides such as Nylon 6 and Nylon 66.Desirable examples of polyolefin fibers are high-density polyethylene,medium density polyethylene, high-pressure method low-densitypolyethylene, linear low-density polyethylene, isotactic polypropylene,ethylene-propylene copolymers, and ethylene copolymers of vinyl monomerssuch as maleic anhydride. Of these, from the perspective of filtermoldability, polyester fibers are desirable. Polyester fibers selectedfrom the group consisting of polyethylene terephthalate,polytrimethylene terephthalate, polybutylene terephthalate, polyethyleneterephthalate isophthalate with an isophthalic acid copolymerizationrate of equal to or more than 20 mol percent, and polyethylenenaphthalate are desirable. So long as the above-stated size issatisfied, one type of synthetic fiber can be used, or two or more typesof synthetic fibers can be combined for use as short fiber A.Commercially available products can also be employed as short fiber A.For example, Nanofront (registered trademark) made by Teijin Fiber is apolyester fiber that can be suitably employed as short fiber A in thepresent invention.

From the perspective of fiber diameter and uniformity of fiber diameter,a composite fiber—having an island-sea structure comprising an islandcomponent comprised of a fiber-forming thermoplastic polymer with anisland diameter of 100 to 1,000 nm and a sea component enclosing theisland component—that is cut and from which the island component is thendissolved away is desirably employed as short fiber A. The islanddiameter can be measured by photographing the cross-section of a singlecomposite fiber by a transmission electron microscope. When the shape ofthe island cross-section is a different shape from a round shape, theisland diameter refers to the diameter of circle circumscribed onto it.

The sea component is desirably comprised of a polymer (referred to as a“readily alkali solution-soluble polymer”, hereinafter) that is morereadily soluble in an alkali solution than the fiber-formingthermoplastic polymer included in the island component. This makes itpossible to remove the sea component by treatment with an alkali. Fromthe perspective of separation of the island, the ratio of thedissolution rate of the readily alkali solution-soluble polymer to thatof the fiber-forming thermoplastic polymer forming the island component(the ratio of their dissolution rates in the alkali solution employed toremove the sea component) is desirably equal to or more than 200,preferably falling in a range of 300 to 3,000. For details regarding thesea component, island component, composite fiber, and method ofmanufacturing the same, reference can be made to WO2008/130019A1, page4, line 21 to page 8, line 24.

In the filter of the present invention, when the proportion of shortfiber A to the total quantity of synthetic fiber is less than 10 masspercent, the fiber obtained performs poorly. At greater than 50 masspercent, the excessive proportion of short fiber A in the filter makesit difficult to achieve an effect by combining binder fiber B and fiberC and increases the airflow resistance, rendering the filter unsuitablefor use as a mask filter. To achieve the stable production of filtersaffording good filter performance, the above proportion is desirably 20to 45 mass percent.

Binder Fiber B

Short fiber A is employed primarily to achieve filter performance.However, because it is so fine, it compromises the sheet-formingproperty when employed alone. Accordingly, in the present invention,binder fiber B is admixed to the filter primarily to enhance thesheet-forming property. Binder fiber B is a synthetic fiber having afiber length ranging from 5 to 10 mm and a fiber diameter ranging from 6to 15 pin. At either a fiber diameter of less than 6 μm or at a fiberlength of less than 5 μm, a large amount of the fiber component dropsout during the sheet-forming step, making it impossible to obtain afilter of adequate strength. When either the fiber diameter exceeds 15μm or the fiber length exceeds 10 mm, it becomes impossible to obtain afilter that performs well. From the perspective of filter performance,the fiber diameter desirably falls within a range of 6 to 13 μm and thefiber length desirably falls within a range of 5 to 7 mm. So long as theabove size is satisfied, a single type of synthetic fiber can be used ortwo or more different types of synthetic fibers can be combined for useas binder fiber B.

Either an unstretched or composite fiber is desirably employed as thesynthetic fiber included in binder fiber B. When employing binder fiberB comprised of an unstretched fiber, a thermocompression step isrequired after drying following sheet-forming. Thus, at leastcalendering or embossing is desirably carried out followingsheet-forming. For details regarding unstretched fibers that can be usedto constitute binder fiber B, reference can be made to WO2008/130019A1,page 9, lines 6 to 12.

Additionally, when binder fiber B is a composite fiber, it is desirablya core-in-sheath composite fiber, comprised of a core portion and asheath portion, that is of suitable strength and flexibility. From theperspective of the strength of the fiber obtained, a core-in-sheathcomposite fiber—in which a polymer component that achieves an adhesiveeffect by fusing when subjected to a heat treatment followingsheet-forming is employed as the sheath portion and another polymer witha melting point 20° C. or more higher than that of the above polymer isemployed as the core portion—is desirable as the core-sheath compositefiber. Binder fiber B can also be a side-by-side composite fiber,eccentric core-in-sheath composite fiber, core-in-sheath compositefiber, or the like in which a binder component (low-melting-pointcomponent) constitutes part or all of the surface of the single fiber.

A copolymer polyester is desirable and an amorphous copolymer polyesteris preferred as the polymer component constituting the sheath portion.Specific examples thereof are random or block copolymers of a componentin the form of an acid such as terephthalic acid, isophthalic acid,2,6-naphthalenedicarboxylic acid, 5-sulfoisophthalic acid sodium, adipicacid, sebacic acid, azelaic acid, dodecanoic acid, or 1,4-cyclohexanedicarboxylic acid and a component in the form of a diol such as ethyleneglycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, diethylene glycol, 1,4-cyclohexanediol, or1,4-cyclohexane dimethanol. Of these, a copolymer polyester, that hasbeen widely employed and is obtained with a principal component in theform of terephthalic acid, isophthalic acid, ethylene glycol, anddiethylene glycol, is desirable in terms of cost. Copolymer polyesterswith glass transition temperatures falling within a range of 50 to 100°C. and not exhibiting clear crystalline melting points are desirable asthe above copolymer polyester. Additionally, examples of the polymerincluded in the core portion are various polyesters such as polyethyleneterephthalate, polytrimethylene terephthalate, and polybutyleneterephthalate. Of these, from the perspective of the performance of thefilter obtained, an example of a preferred binder fiber B is acore-in-sheath composite fiber in which the core portion is polyethyleneterephthalate and the sheath portion is a copolymer polyester.

Fiber C

The short fiber A is employed primarily to achieve filter performanceand binder fiber B is employed primarily to achieve good sheet-formingproperty. Mixing in fiber C that is of greater fiber length and fiberdiameter than short fiber A and smaller fiber length and fiber diameterthan binder fiber B makes it possible to obtain a mask filter thataffords both good particle filtration efficiency and low pressure lossby folding up the synthetic fibers containing the short fibers into aconcentric circular pleat form. In contrast, when a mask filter isfabricated by forming fiber A alone, or a mixture of fiber A and fiberB, into a sheet and folding up it into a concentric circular pleat form,separation or sticking occurs in the process of removal from thesheet-forming machine or folding machine, making it difficult to obtaina filter of desired shape. To obtain a filter of desired shape, fiber Cwith a fiber diameter of greater than 1 μm and less than 7 μm and with afiber length of equal to or more than 2 mm but less than 5 mm isdesirably employed, and one with a fiber diameter of equal to or morethan 2 μm and equal to or less than 5.5 μm and with a fiber length ofequal to or more than 2 mm and equal to or less than 4 mm is preferablyemployed.

The fiber C is not specifically limited other than that it be asynthetic fiber of the above-stated size. From the perspective of filtermoldability and the like, polyamide fibers, polyester fibers, polyolefinfibers, and the like are desirable. Of these, polyester fibers arepreferred. Examples of polyester fibers are those given by way ofexample for short fiber A above. In particular, the use of fiber C inthe form of a polyester fiber is desirable from the perspective offilter performance when short fiber A is a polyester fiber. One type ofsynthetic fiber can be employed, or two or more different types ofsynthetic fibers can be employed, as fiber C.

The proportion of fibers other than short fiber A, that is, theproportion of the total quantity of binder fiber B and fiber C,desirably constitutes 50 to 90 mass percent, preferably 55 to 80 masspercent, of the synthetic fibers included in the filter of the presentinvention. Employing fiber B and fiber C within the stated range makesit possible to achieve a good effect by using the two fibers incombination with short fiber A. From the perspective of furtherenhancing the effect achieved by using both binder fiber B and fiber C,the proportion of the synthetic fiber accounted for by binder fiber B isdesirably 30 to 60 mass percent, and the proportion accounted for byfiber C is desirably 15 to 45 mass percent.

The method of manufacturing a mask filter of the present invention willbe described next.

The mask filter of the present invention can be manufactured by forminga sheet out of synthetic fibers in the form of a mixture of short fiberA, binder fiber B, and fiber C and folding up the sheet into aconcentric circular sheet form. Specifically, the synthetic fibers aredispersed in a solvent (normally water) to prepare a slurry, the slurrythat has been prepared is formed into a sheet with a sheet-formingmachine, and then, while still wet, the nonwoven fabric is transferredto a folding machine and folded. Subsequently, the filter that has beenfolded up in the folding machine is received in a receiving can. Thefilter is discharged from the receiving can and dried in a dryingmachine to obtain a mask filter that has been folded up into aconcentric circular pleat form.

The slurry can be prepared by known methods using the above syntheticfibers. Water is normally employed as the solvent. As needed, additivessuch as dispersing agents and antifoaming agents can also be employed.

Any sheet-forming machine that is normally employed to manufacture wetnonwoven fabric can be employed without limitation. The shape and sizeof the folding machine that folds up the nonwoven fabric that istransferred from the sheet-forming machine can be determined based onthe desired mask shape and size. When the sheet-forming machine ispositioned over the folding machine following sheet-forming, the wetnonwoven fabric covering the walls in the interior of the sheet-formingmachine can be adhered to the folding machine to cover the outer surfaceof the folding machine with the nonwoven fabric. Here, the nonwovenfabric is wet and fluid, and will thus adhere to the outer surface ofthe folding machine in close conformity to the outer surface shape ofthe folding machine.

Subsequently, the folding machine, the outer surface of which has beencovered by the wet nonwoven fabric, folds up. For example, using afolding machine of cylindrical shape that is capable of folding up innested fashion, it is possible to fold up the wet nonwoven fabric intoconcertinas. Subsequently, the wet mask filter is received from thefolding machine by the receiving can and transferred to the dryingmachine. The wet mask filter is desirably discharged from the receivingcan into a drying machine and dried to obtain a mask filter of desiredshape. FIG. 1 (a perspective view) shows an example of a mask filterthus formed.

The mask filter that has been obtained by the above steps is folded upinto a concentric circular pleat shape, and thus affords high particlefiltration efficiency and a low pressure loss. Specifically, the presentinvention can achieve a filter performance in the farm of a DOP (dioctylphthalic acid) particle filtration efficiency of equal to or higher than80 percent (further, equal to or higher than 90 percent, and desirably,95 percent to 100 percent), and a pressure loss of equal to or less than300 Pa (further equal to or less than 250 Pa, desirably 50 to 220 Pa).In the present invention, the phrases “DOP particle filtrationefficiency” and “pressure loss” refer to values measured by the methodsdescribed in Examples further below.

An example of a desirable filter shape in terms of achieving the abovefilter performance is one in which the filter thickness falls within arange of 0.4 to 1.5 mm, the pleat height falls within a range of 5 to 30mm, the outer diameter of the pleat at the outermost circumferenceposition falls within a range of 50 to 120 mm, and the total number ofpleats falls within a range of 6 to 15. The shape can be adjusted bymeans of the slurry concentration, shape of the folding machine, andmanufacturing conditions. As set forth above, it is difficult to obtaina filter of desired shape without combining fibers A to C. In contrast,in the present invention, the synthetic fiber combining the abovemultiple fibers is employed to obtain a filter of the above-describeddesired shape. It is also desirable for the total mass of the filter tofall within a range of 2 to 12 g and for the mass per unit area to fallwithin a range of 80 to 180 g/m² when applied to a mask. In the presentinvention, the term “mass per unit area” is a value measured accordingto JIS P8124 (Paper-Determination of grammage), and the filter thicknessis a value measured according to JIS P8118 (Paper andboard-Determination of thickness and density).

The mask filter of the present invention as set forth above is suited tovarious mask filters used in particle-trapping applications such as dustmasks. That is, the present invention also relates to aparticle-trapping mask comprising the mask filter of the presentinvention. The mask of the present invention is suited to replaceabledust masks, and the filter of the present invention can be employed as areplaceable filter.

FIG. 2 shows an example of the mask of the present invention. The dustmask shown in FIG. 2 is comprised of, at a center thereof, a filterelement 1 in the form of the filter of the present invention, a cap 2positioned to the outside thereof, and a holder 3 positioned to theinside thereof, thereby constituting an integrated filter member. Onthis filter member are mounted a face-contact member 4, an exhalationvalve 5, and an inhalation valve 6, thereby constituting the mask mainbody. The packing 32 of holder 3 plays the role of enhancing thetightness of fit. Strap fastener 8 is mounted through thestrap-fastening mount 7 of holder 3 to constitute a dust mask. Whenwearing the dust mask, it suffices to mount the head band 10 of strapfastener 8 on the head and adjust the length of the strap fastener withbuckle 9. In the dust mask shown in FIG. 2 in the form of a replaceabledust mask, pressing knob 31 of holder 3 to release cap 2 permitsreplacement of filter element 1.

The present invention further relates to a method of manufacturing themask filter of the present invention. The manufacturing method of thepresent invention is characterized by folding up the nonwoven fabricinto a concentric circular pleat form while in a wet state. The detailsof the manufacturing method of the present invention are as set forthabove. According to the present invention, the synthetic fibers with theabove combination can be used to obtain a mask filter in which wetnonwoven fabric containing short fibers is folded up into a concentriccircular pleat form.

EXAMPLES

The present invention will be further described below through Examples.However, the present invention is not limited to the embodiments shownin Examples.

The synthetic fibers employed in Examples and Comparative Examples wereas follows. The fiber diameters indicated below are values (averagevalues, n=5) measured by photographing a fiber cross section at30,000-fold magnification by a transmission electron microscope (TEMwith a length-measuring function). The fiber diameters referred toherein are the diameters of circles circumscribed onto single fibercross-sections. The fiber lengths are values (average values, n=5)measured at a magnification of 20 to 500-fold with the fiber beingmeasured having been positioned on a base.

Short fiber A-1: Polyethylene terephthalate fiber (Nanofront (registeredtrademark) made by Teijin Fiber) with a fiber diameter of 703 nm and afiber length of 0.8 mm

Fiber B-1: Core-in-sheath composite fiber (core portion: polyethyleneterephthalate; sheath portion: copolymer polyester) with a fiberdiameter of 6.8 μm and a fiber length of 5 mmFiber B-2: Core-in-sheath composite fiber (core portion: polyethyleneterephthalate; sheath portion: copolymer polyester) with a fiberdiameter of 10.1 μm and a fiber length of 5 mmFiber B-3: Core-in-sheath composite fiber (core portion: polyethyleneterephthalate; sheath portion: copolymer polyester) with a fiberdiameter of 12.5 μm and a fiber length of 5 mmFiber C-1: Polyethylene terephthalate fiber with a fiber diameter of 2.4μm and a fiber length of 3 mmFiber C-2: Polyethylene terephthalate fiber with a fiber diameter of 5.2μm and a fiber length of 3 mmGlass fiber: Glass fiber with a fiber diameter of 0.5 to 1.80 μm

Examples 1 to 9

Thirty filters were fabricated by the following method.

A quantity of short fiber A-1 corresponding to 30 filters was weighedout and charged to a vessel. A dispersing agent (DT-100, made byMomentive Performance Materials, Inc. (Japan)) was added, after whichwater was gradually added. A defoaming agent (TSA-730, made by GEToshiba Silicones Co., Ltd.) was then added and slurry 1 was prepared.The concentration of dispersing agent in slurry 1 was about 1.4 masspercent. The concentration of defoaming agent was about 0.7 masspercent. The concentration of the solid component in slurry 1 was about6.4 mass percent. Separately, quantities corresponding to 30 filters ofthe fiber corresponding to fiber B and the fiber corresponding to fiberC shown in Table 1 were weighed out, charged to a vessel, mixed, anddivided into 30 equal portions, each of which was charged to a separatevessel. Slurry 1 was divided into 30 equal portions, each of which wasseparately charged to one of the vessels, suitable quantities of waterwere added, and the mixtures were stirred with mixers to prepare aslurry 2. The concentration of the solid component of slurry 2 was about0.3 to 0.6 mass percent.

The slurry 2 that had been thus prepared was formed into a sheet in asheet-forming machine. Following sheet formation, the wet nonwovenfabric was transferred from the sheet-forming machine onto a cylindricalfolding machine capable of folding in nested fashion. It was then foldedup into a concentric circular pleat shape and dried at 90 to 160° C. tofabricate a filter.

The number of pleats, pleat height, and outer diameter of the pleat atthe outermost circumference position were varied by changing the foldingmachine employed. The filter thickness was adjusted by means of theslurry concentration.

Comparative Example 1

With the exception that short fiber A-1 and fiber B-2 were employedwithout a fiber corresponding to fiber C, a slurry 3 was prepared by thesame method as in the above Examples. The concentration of the solidcomponent in slurry 3 that was prepared was about 0.3 mass percent.

Using slurry 3 thus prepared, an attempt was made to fabricate a filterby folding up into a concentric circular pleat form by the same methodas in the above Examples. Although an attempt was made following dryingto remove the filter from the folding machine, it stuck firmly to thefolding machine and ended up having to be peeled off, precluding theobtaining of a filter.

Comparative Examples 2 to 4

Quantities of glass fiber and fiber B-3 corresponding to a single filterwere weighed out, charged with a suitable quantity of water to a mixer,and stirred to prepare slurry 4. The concentration of the solidcomponent in slurry 4 thus obtained was about 0.1 to 0.5 mass percent.

A filter was fabricated by the same method as in the above Examples fromslurry 4 thus prepared.

Comparative Examples 5 to 9

A slurry 5 was prepared from the combination of fibers shown in Table 2by the same method as in Examples 1 to 9. The concentration of the solidcomponent in slurry 5 thus obtained was about 0.5 to 0.7 mass percent.

Slurry 5 thus prepared was used to form a sheet on a round net, peeledoff the net, and dried at 90 to 160° C. to prepare a sheet-like filter.The thickness of the filter was adjusted by means of the slurryconcentration.

Evaluation Methods

(1) DOP Particle Filtration Efficiency

The DOP particle filtration efficiency of the filter fabricated inExamples and Comparative Examples was measured by the following methodwith an Automated Filter Tester Model 8130 made by TSI.

The concentration of dioctyl phthalate was measured before and afterpassing air containing particles of dioctyl phthalate (DOP) with amedian diameter of the particle distribution of 0.15 to 0.25 μm(geometric standard deviation of ≦1.6 μm) and a 100 mg/m³ concentrationthrough the filter at a flow rate of 85 L/min, and the DOP-trappingefficiency was calculated based on the following expression:

(DOP concentration after passage of sample)/(DOP concentration beforepassage of sample)×100 percent

(2) Pressure Loss

Air was passed through the filters fabricated in Examples andComparative Examples at a rate of 30 L/min and the pressure differentialbefore and after passage of the sample was measured as the pressureloss.

TABLE 1 Composition (mass %) Corresponding Corresponding to Corresponingto to short fiber A fiber B fiber C Fiber B + Glass Short fiber A-1Fiber B-1 Fiber B-2 Fiber B-3 Fiber C-1 Fiber C-2 Fiber C fiber PleatFiber diameter portion Pleat 703 nm 6.8 μm 10.1 μm 12.5 μm 2.4 μm 5.2 μm— — Thick- Number outer portion Fiber length ness of diameter height No.0.8 mm 5 mm 5 mm 5 mm 3 mm 3 mm — — (mm) pleats (mm) (mm) Ex. 1 27 — 55— — 18 73 — 0.8 6 61 17 Ex. 2 22 — 39 — — 39 78 — 1 6 61 17 Ex. 3 22 45— — 11 22 78 — 0.5 9 68 19 Ex. 4 26 43 — — 10 21 74 — 0.6 9 68 19 Ex. 530 40 — — 10 20 70 — 0.6 9 68 19 Ex. 6 32 39 — — 10 19 68 — 0.6 9 69 19Ex. 7 34 38 — —  9 19 66 — 0.7 9 69 19 Ex. 8 35 37 — —  9 19 65 — 0.8 1279 22 Ex. 9 42 33 — —  8 17 58 — 1 12 79 22 Comp. Ex. 1 33 — 67 — — — 67— Fabrication couldn't be done. Comp. Ex. 2 — — — 44 — — 44 56 0.9 6 6617 Comp. Ex. 3 — — — 39 — — 39 61 0.7 9 72 19 Comp. Ex. 4 — — — 40 — —40 60 0.6 12 61 9

TABLE 2 Composition (mass %) Corresponding Corresponding Correspondingto to short fiber A to fiber B fiber C Fiber B + Short fiber A-1 FiberB-1 Fiber C-1 Fiber C-2 Fiber C Fiber diameter 703 nm 6.8 μm 2.4 μm 5.2μm — Fiber length Thickness Diameter No. 0.8 mm 5 mm 3 mm 3 mm — (mm)(mm) Comp. Ex. 5 22 45 11 22 78 0.8 80 Comp. Ex. 6 26 43 10 21 74 1 80Comp. Ex. 7 30 40 10 20 70 0.5 80 Comp. Ex. 8 32 39 10 19 68 0.6 80Comp. Ex. 9 34 38 9 19 66 0.6 80

TABLE 3 DOP Mass filtration Pressure per unit Number efficiency lossMass area No. Shape of pleats (%) (Pa) (g) (g/m²) Ex. 1 Concentric 699.659 157 2.54 113.0 Ex. 2 circular pleat 99.986 217 3.59 159.7 Ex. 3form 9 99.842 81 3.39 94.0 Ex. 4 99.949 102 3.62 100.1 Ex. 5 99.983 1393.84 105.0 Ex. 6 99.992 148 3.92 106.6 Ex. 7 99.998 179 4.16 113.0 Ex. 812 99.997 128 8.29 134.6 Ex. 9 99.999 132 9.72 157.9 Comp. 9 Measurement— — Ex. 1 couldn't be done. Comp. 6 99.992 157 1.95 87 Ex. 2 Comp. 999.979 144 3.29 146 Ex. 3 Comp. 12 99.924 83 3.82 112 Ex. 4 Comp. Singlesheet — 99.155 435 0.62 97 Ex. 5 Comp. 99.579 514 0.66 104 Ex. 6 Comp.99.849 626 0.69 109 Ex. 7 Comp. 99.893 681 0.71 111 Ex. 8 Comp. 99.971830 0.77 121 Ex. 9

Evaluation Results

As set forth above, in Comparative Example 1, in which an attempt wasmade to fabricate a filter by folding up into a concentric circularpleat form without employing a fiber corresponding to fiber C, no filtercould be obtained.

In contrast, in Examples 1 to 9, filters of desired shapes havingconcentric circular pleats were successfully fabricated. As shown inTable 3, all of the filters of Examples had DOP particle-trappingefficiencies exceeding 99 percent, indicating an extremely goodparticle-trapping capacity. A comparison with the single sheet-likefilters of Comparative Examples 5 to 9 confirmed that the use of aconcentric circular pleat shape achieved both high particle filtrationefficiency and a low pressure loss. Further, comparison with ComparativeExamples 2 to 4 revealed that Examples 1 to 9 yielded filters made ofsynthetic fibers affording filter performance (particle filtrationefficiency and pressure loss) equivalent to or better than that offilters employing glass fiber. When the filters in which glass fiber wasemployed were incinerated, the glass remained as a residue in the wasteproduct. By contrast, since the synthetic fiber filters could becompletely incinerated, they were desirable from an environmentalperspective. Further, since glass fiber is brittle and is not ofconstant fiber diameter, it tends not to provide a stable filter ofconstant quality. In contrast, since synthetic fibers readily yield bothuniform fiber diameters and fiber lengths, the use of the syntheticfibers makes it possible to stably provide filters of desiredperformance.

The present invention is useful in fields such as the manufacturing ofreplaceable dust mask filters.

1. A mask filter, wherein wet nonwoven fabric comprised of syntheticfiber is folded up into a concentric circular pleat form, and thesynthetic fiber comprises: 10 to 50 mass percent of short fiber A havinga fiber length ranging from 0.1 to 1 mm and a fiber diameter rangingfrom 100 to 1,000 nm, and further comprises: binder fiber B having afiber length ranging from 5 to 10 mm and a fiber diameter ranging from 6to 15 μm, and fiber C of greater fiber length and fiber diameter thanshort fiber A and smaller fiber length and fiber diameter than binderfiber B.
 2. The mask filter according to claim 1, wherein the syntheticfiber comprises 50 to 90 mass percent of a total quantity of binderfiber B and fiber C.
 3. The mask filter according to claim 1, which hasa DOP particle filtration efficiency of equal to or greater than 80percent and a pressure loss of equal to or less than 300 Pa.
 4. The maskfilter according to claim 1, which has a mass per unit area of 80 to 180g/m².
 5. The mask filter according to claim 1, which has a filterthickness ranging from 0.4 to 1.5 mm, a pleat height ranging from 5 to30 mm, a pleat outer diameter at the outermost circumference positionranging from 50 to 120 mm, and a total number of pleats ranging from 6to
 15. 6. The mask filter according to claim 1, which has a total weightranging from 2 to 12 g.
 7. The mask filter according to claim 1, whereinthe short fiber A is a polyester fiber.
 8. The mask filter according toclaim 1, wherein the binder fiber B is a core-in-sheath composite fiberwith a core portion of polyethylene terephthalate and a sheath portionof copolymer polyester.
 9. The mask filter according to claim 1, whereinthe fiber C is a polyester fiber.
 10. A method of manufacturing the maskfilter according to claim 1, which comprises folding up the nonwovenfabric into a concentric circular pleat form while in a wet state.
 11. Aparticle-trapping mask comprising the mask filter according to claim 1.